Control of a multi-carrier power amplifier

ABSTRACT

A method of operating a Time Division Multiple Access (TDMA) communication system is disclosed. The system includes a multi-carrier power amplifier. The method includes the steps of logging call activity on each timeslot of each carrier associated with the multi-carrier power amplifier; logging total transmit power of the multi-carrier power amplifier for each timeslot; and logging the data rate of any data calls being carried by the multi-carrier power amplifier. Transmission power is made available for a new call by reducing the transmission power of a data call.

[0001] This invention relates to a method and apparatus for dimensioningand controlling a multi-carrier power amplifier. It finds particular,but not exclusive use in a Base Transceiver Station (BTS) in a GSM basedcommunication system adapted to use the EDGE data transmission standard.

[0002] Traditionally, GSM BTSs have employed single-carrier poweramplification schemes. That is, a single amplifier chain was providedfor each GSM carrier signal. Each GSM carrier signal comprises 8 timeslots per frame, and is thus theoretically capable of supporting 8conversations or data connections simultaneously.

[0003] Typically, a BTS comprises several transceiver units, each onehaving a dedicated power amplifier (PA). FIG. 1 shows a BTSconfiguration according to the prior art. In this example, eighttransceiver units (TRX) are provided 10 a-10 h. Each transceiver unithas an associated power amplifier (PA) 20 a-20 h. The Power Amplifier isresponsible for boosting the output power of the TRX to a suitable levelfor transmission. However, the resultant signals from each of the PAshave to be combined in order to route them to a common transmissionantenna. This requires the use of a high power combiner 30, which hasthe drawback that a considerable amount of the input power is dissipatedin the combining process. Typically, 3.2 dB is dissipated in heat per2-way combine in a hybrid combiner. As the output of eight TRXs needcombining, there are 3 combining stages needed, resulting in a loss ofnearly 10 dB, or 90% of the amplified signal. This places constraints onthe BTS in the fields of power regulation and thermal design.

[0004] The configuration is actually more complicated due to theprovisions made for the receive path from the antenna, but we are onlyconcerned with the transmission path here.

[0005] It is now possible to implement PAs which are capable ofamplifying more than a single carrier signal. These are known asMulti-Carrier Power Amplifiers (MCPAs) or Multi-Carrier Linear PowerAmplifiers (MCLPAs). Such a configuration is shown in FIG. 2. The TRXs10 a-10 h are identical to those shown in FIG. 1, and each supports asingle GSM carrier as before. The outputs from the TRXs are nextcombined in a low power combiner 50. The relative losses in this arestill of the order of 10 dB in total, but as the input power to thecombiner is considerably lower, the absolute power loss is much lower.

[0006] The output of the combiner 50 is next fed into the input of theMCPA 60. The MCPA is a wideband linear amplifier which, in thisinstance, is capable of amplifying the outputs of all eight TRXssimultaneously, before transmitting the signals via antenna 40.

[0007] One of the problems which has thus far held back deployment ofMCPAs is the linearity which is required by the GSM specifications. GSMspecification 05.05 Section 4.2.1, “Spectrum due to the modulation ofwideband noise”, particularly sets the limits on the acceptable levelsof noise products due to non-linearity effects in the PA. Only recentlyhas it been possible to implement MCPAs which meet all the necessarycriteria laid out in the GSM specifications.

[0008] Defining the transmit power output requirement for a singlecarrier PA (SCPA) is straightforward. The SCPA is capable of supportingup to eight simultaneous connections—one on each of the timeslots whichmake up a GSM frame. Each timeslot is processed in turn, and so themaximum output power required form the SCPA is equivalent to the maximumpower called for in any one of the timeslots. Defining the power outputrequirement for an MCPA can be more problematic. An assumption made inspecifying the power output requirement for an MCPA is that if it isoperating substantially linearly, then the total output power on a giventimeslot is given by the sum of the individual powers of each carrier.$P_{TOT} = {\sum\limits_{0}^{n}\quad P_{n}}$

[0009] where n=number of GSM carriers being amplified, which is eight inthe example cited in FIG. 2.

[0010] A simple means of defining the maximum power required would be toassume that each individual carrier was operating at its maximum level,so that the total power output required is equal to the sum of eachindividual maximum power requirement, or

P _(TOT) =n·P _(MAX)

[0011] While this solution will produce an MCPA which will support eachGSM carrier signal successfully under all conditions, it will be greatlyover-specified, and will consume excessive amounts of energy. Forexample, linear PAs tend to be only 5-7% efficient, or to put it anotherway, 93-95% of the energy supplied is dissipated as heat. Thisinefficiency is due to the mode in which the amplifiers need to operatein order to meet the linearity specifications. The net result is thateven if the transmitter is required to provide a relatively low amountof output RF power, or even no output at all, the amplifier remainsbiased in such a way that large amounts of heat have to be dissipated.For example, assuming that an MCPA supports eight 1 W carriers, then itmust be designed to dissipate between 100 W and 150 W of heat energy.

[0012] Not only do such requirements impose difficult design constraintson BTSs in terms of heat dissipation, but component failures increasewith such increases in temperature, posing reliability problems. Theadded charges for electricity also become significant when appliedacross an entire cellular network.

[0013] According to a first aspect of the present invention, there isprovided a method of operating a Time Division Multiple Access (TDMA)communication system comprising a multi-carrier power amplifier (MCPA),comprising the steps of: logging call activity on each timeslot of eachcarrier associated with the multi-carrier power amplifier; logging totaltransmit power of the multi-carrier power amplifier for each timeslot;and logging the data rate of any data calls being carried by themulti-carrier power amplifier, wherein transmission power is madeavailable for a new call by reducing the transmission power of a datacall.

[0014] According to a second aspect of the present invention there isprovided a communication system comprising: a multi-carrier poweramplifier; and a database comprising: information concerning callactivity on each timeslot of each carrier associated with themulti-carrier power amplifier; information concerning total transmissionpower of the multi-carrier power amplifier for each timeslot; andinformation concerning the data rate of any data calls supported by themulti-carrier power amplifier wherein the multi-carrier power amplifieris arranged to reduce the transmission power associated with a data callin order to make transmission power available for a new call.

[0015] Advantageously, the method and system may be applied to anycommunication system adapted to use the EDGE standard. EDGE iseffectively an overlay which may be used with TDMA based communicationsystems such as GSM or IS-136. It uses substantially the same hardwareand protocols, but uses a different modulation scheme to achieve higherdata rates.

[0016] Advantageously, the realisation that the data rate on aparticular call can be reduced, to free up transmission power for thenew call, means that the new call can be accommodated at the expense ofa potentially small, and possibly temporary, drop in the data rate of asingle call.

[0017] Preferably, the assignment of timeslots to calls proceeds suchthat there is minimal variation in transmit power from one timeslot tothe next. This ensures that should there be a demand for increasedtransmit power for any one call, then sufficient transmit power capacityshould be available across all timeslots.

[0018] Preferably, if a call data rate has to be reduced to accommodatea new call, then the data call currently having the highest data ratewill have its rate reduced, via reduced transmit power, before othercalls. As the call parameters such as transmit power and data rate arecontinuously monitored, this test is continuously re-evaluated.

[0019] In an advantageous development, each call may have an assignedpriority level. This may be due to the particular charging tariff towhich a subscriber belongs, or it may be that a user can elect to pay apremium fee on a call by call basis to ensure that a high speed dataconnection is maintained for that call. Other scenarios exist wherecalls can be prioritised. Call priority level can be used to determinewhich of the currently supported calls can have its data rate reduced,with lower priority calls being affected first. The incoming call whichis attempting call set up may also have a priority level which willinfluence how timeslots are allocated to it.

[0020] Calls may have associated with them a minimum data rate, belowwhich they will not drop. In a preferred embodiment where EDGE is usedto achieve higher data rates, then the minimum data rate corresponds tothe data rate which could be achieved if the modulation scheme of thenetwork underlying the EDGE standard were used. In the case of a GSMnetwork, GMSK would be used to modulate the data. Using this modulationscheme, the data rate would be 9.6 Kbit/s.

[0021] In a case where the MCPA is not operating at maximum transmitpower on each timeslot, but it has insufficient free power available onany one timeslot to be able to support a new call, then it is possibleto reallocate the current timeslot assignments in such a way that whatspare transmit power is available is reallocated to the same timeslot.This enables a carrier using that timeslot to support the new call.

[0022] For a better understanding of the present invention, and tounderstand how the same may be brought into effect, the invention willnow be described, by way of example only, with reference to the appendeddrawings in which:

[0023]FIG. 1 shows some elements of the transmitter chain in a BTSutilising single carrier power amplifiers according to the prior art;

[0024]FIG. 2 shows some elements of the transmitter chain in a BTSutilising a multi-carrier power amplifier according to the prior art;

[0025]FIG. 3 shows a graph of achievable data rate in an EDGE systemrelated to C/I;

[0026]FIG. 4 shows a flowchart detailing operation of an embodiment ofthe invention; and

[0027]FIG. 5 shows a flowchart detailing operation of another embodimentof the invention.

[0028] The current GSM network is going to be used increasingly for datarather than voice calls in the near future. Indeed, it is estimated thateventually, data calls by users browsing the World Wide Web, forinstance, will eventually greatly outnumber voice calls over cellularnetworks.

[0029] This fact in conjunction with the increasing desire to use MCPAscreates new problems in call-management. Specifically, more timeslotsare likely to be allocated to a single user, and the charging for use ofthe network may well develop along the line of charges per byte of datatransferred, rather than connection time as is the norm at present.

[0030] The increasing use of GSM cellular networks for data traffic hasan impact on power transmission requirements. This is particularly truein the EDGE system. EDGE is a standard which supplements TDMA systems,such as GSM, and allows relatively high speed data connections to beestablished using existing network infrastructure. The high speed isachieved via use of a different modulation scheme. GSM uses GaussianMinimum Shift Keying (GMSK) and EDGE uses 8-PSK. To boost the data ratefurther, several timeslots can be used by the same user for datatransfer. A drawback of EDGE is that its modulation scheme requires ahigher Carrier to Interference Ratio (C/I) than GSM in order to operatereliably. GSM typically requires a C/I of 9 dB, whereas EDGE needsapproximately 20 dB more than this.

[0031] Broadly speaking, there are two ways to increase C/I. The firstway is to reduce the interference power (I). However, this is largelyout of the control of the operator of the cellular system, as it isderived from other systems, and indeed other signals within his system.The second way is to increase the carrier power (C). However, thisplaces demands on the transmission equipment, and goes on to causeinterference to other systems, and so is not a viable option beyond acertain limit.

[0032] In EDGE systems, it has been found empirically that there is adirect relationship between achievable data rates and C/I. FIG. 3 showsa graph representing empirical studies into the relationship. It can beseen that higher data rates are available for higher C/I values. This isto be expected, as a clearer signal mitigates the need forre-transmission of data which is lost due to poor connection quality.However, it can be seen that the relationship is not linear, and thatincreasing C/I linearly does not cause the data rate to increasesimilarly. The maximum achievable data rate tails off at higher valuesof C/I.

[0033] If the MCPA is running near its maximum output power, and a newvoice or data call is initiated, or a handover is attempted into thecell it is serving, then a new timeslot on a carrier needs to beassigned. In the case where a BTS is equipped with SCPAs, the onlyfactor that needs to be considered in determining whether a call can beaccepted is whether a spare timeslot exists on any of the SCPAs. If atimeslot is available, the call is set up. In the case where an MCPA isused, there may well be one or more spare timeslots available, but nowthe factor which needs to be considered is whether there is sufficienttransmit power available to support the new call.

[0034] If the MCPA is supporting a given number of data and voice calls,and the MCPA is operating at its maximum transmit power on alltimeslots, one option is to refuse the new call set up. If the new callwas an attempted handover, it might be transferred to anotherneighbouring cell. If that is not possible, it may stay connected to itscurrent cell. If that is not possible, then the call will be dropped.None of these scenarios is particularly desirable.

[0035] A preferable embodiment of the invention continuously monitorsthe allocation of timeslots, the nature of the call on each timeslot,and the data rate being achieved for each data call. If an attempt ismade to set up a new call when the MCPA is operating at maximum transmitpower on all timeslots, or at such a level that insufficient transmitpower is available to support the new call, then there are severaloptions which can be pursued before call set up is refused. TS0 TS1 TS2TS3 TS4 TS5 TS6 TS7 TRX0 −2 dB −8 dB −4 dB −10 dB −4 dB D:32 V V V VTRX1 −12 dB  −12 dB  −6 dB   0 dB −4 dB V V D:30 V V TRX2 −4 DB −6 dB −2dB −6 dB −12 dB V TRX3 −8 DB −8 dB  −6 dB −8 DB V V D:24 V TRX4 −4 dB −2dB −6 dB V D:26 V TRX5 −8 dB −8 dB −2 dB V V D:30 TRX6 −6 dB −8 dB V VTRX7 −2 dB   0 dB −8 dB  −4 dB −10 dB  V V V V V

[0036] An example of a database that is maintained by the system foreach MCPA is given above. It shows which timeslots are active on whichTRX, what the power level associated with the call is, and whether thecall is data or voice. If it is a data call, the data speed is recordedalso. ‘V’ signifies a voice call, and ‘D’ signifies a data call, withthe number following being the data rate in Kbit/s. Blank cells in thetable signify no call activity on that carrier/timeslot pair.

[0037] Firstly, if one or more of the calls being supported by the MCPAis a data call, then in order to free up some transmission power tosupport the attempted new call, the transmission power associated withone of the data calls can be reduced by an amount equal to the amountneeded to support the new call.

[0038] The drop in transmission power for the present data call willresult in a drop in the achievable data rate, as shown in FIG. 3.

[0039] Preferably, if there is more than one data call in progress, thenthe data call having the highest rate is reduced first.

[0040] Using the graph of FIG. 3, it can be seen that an EDGE callhaving a data rate of 32 Kbit/s can free up enough power for anothercall to operate at a transmission power of −4 dB if its transmissionpower is dropped by a corresponding amount. Consequently, the data rateexperienced by that user drops to 26 Kbit/s. Such a drop in data ratewill have an effect on the user of the MS making that call, but it willnot be so dramatic as dropping the potential new call.

[0041] In the tabular example above, the data call on TRX0 at TS2 has adata rate of 32 Kbit/s and a transmit power level of −2 dB. As this callhas the highest data rate, an embodiment of the invention would liberatesome power on TS2 by reducing the transmit power associated with TRX0.

[0042] In any event, under the EDGE standard, the data rate will drop asthe user experiences a weaker signal from the BTS. This means that atthe edge of the cell, or in other areas of poor reception, the data rateachievable will be lower than if the MS were receiving a strongersignal. In the event that the signal reaching the MS is insufficient tomaintain an EDGE connection, the modulation scheme switches to that ofthe underlying network, for instance, GSM. This offers a lower datarate, but one that is robust and generally guaranteed. In the GSM case,using GMSK modulation, the data rate available is 9.6 Kbit/s.

[0043] The level to which trade-offs between data rate and callconnection occurs is determined by the system operator on the basis ofwhich event is perceived to cause most inconvenience to users of thenetwork. It is normally preferable to maintain a call, even if the datarate is reduced, so that a new call can be accommodated, than to refusea call set up or to drop a call because it is not possible to hand itover to a new cell.

[0044]FIG. 4 shows a flowchart which summarises operation of anembodiment of the invention.

[0045] At box 100, an attempt is made to set up a new call or hand overan existing call from a neighbouring cell. This can be a voice or a datacall.

[0046] A check is made at 110 to determine whether sufficient transmitpower is available in order to set up the call. This check is performedon the basis of the contents of the database which records call activityfor a given MCPA.

[0047] If the call can be set up using the currently available freetransmit power capacity, then call set up occurs as shown at 120.

[0048] If there is insufficient transmit power capacity to set up thecall, the next check 130 determines whether a data call is listedamongst the currently supported calls.

[0049] If there is one or more data calls in progress then one of themwill have their transmission power reduced 140. In a preferredembodiment, the data call currently enjoying the highest data rate isselected to have its transmission power reduced ahead of any other datacalls. In any event, the transmission power of any given data call willnot be reduced below a guaranteed minimum.

[0050] Once this has been done, and sufficient transmission power hasbeen freed up for the new call, the new call is set up 120.

[0051] If there are no data calls in progress, then it is necessary torefuse call set up of the new call 150.

[0052] Secondly, if, for example, two time slots have some sparetransmit power capacity, but not enough to support the new callindividually, then the timeslot allocations of the currently supportedcalls can be re-arranged so that the two portions of spare capacity canbe re-allocated on the same timeslot so that the new call can be set upon that timeslot.

[0053] This will incur some delay in the call set up process while theexisting timeslot allocations are re-ordered, but this delay will not benoticeable to a user.

[0054] Thirdly, it may be possible in the future to attach priorities tocertain call types. This may be based on any number of different factorssuch as the charging tariff to which a customer belongs, whether a callis data, voice or fax, or whether a user has selected to pay acall-by-call premium to enjoy a high data rate. It is possible to assigncertain users the lowest possible priority so that they can effectivelyonly make emergency calls. If the calls being supported by the MCPAcontain a number of calls of differing priority levels, then thedecision on which call has its data rate reduced may be made based onthe relative priority levels of all currently supported calls.

[0055] This is summarised in FIG. 5, which shows a flowchart similar tothat of FIG. 4. All steps are identical except that step 140 is replacedby step 240 whereby the data call having the lowest assigned priorityhas its transmission power reduced before any other data call.

[0056] One or more of the embodiments herein described can be combinedwith one or more of the other embodiments herein described.

[0057] These measures ensure that the maximum number of calls ismaintained by the communication network. The result may be reduced datarate for some users in some circumstances, but it will allow more usersaccess to the network, which is generally preferable to calls beingdropped at handover, or not set up in the first instance.

[0058] In the light of the foregoing description, it will be clear tothe skilled man that various modifications may be made within the scopeof the invention. In particular, the EDGE standard may be replaced withanother variable data rate transmission standard. The underlying networkhas been described in terms of GSM, but the skilled man will realisethat any suitable TDMA system could be used instead.

[0059] The present invention includes any novel feature or combinationof features disclosed herein either explicitly or any generalisationthereof irrespective of whether or not it relates to the claimedinvention or mitigates any or all of the problems addressed.

1. A method of operating a Time Division Multiple Access (TDMA)communication system comprising a multi-carrier power amplifier,comprising the steps of: logging call activity on each timeslot of eachcarrier associated with the multi-carrier power amplifier; logging totaltransmit power of the multi-carrier power amplifier for each timeslot;and logging the data rate of any data calls being carried by themulti-carrier power amplifier, wherein transmission power is madeavailable for a new call by reducing the transmission power of a datacall.
 2. A method as claimed in claim 1 wherein the communication systemoperates according to the EDGE standard.
 3. A method as claimed in claim1 or 2, wherein timeslots are assigned such that the total transmissionpower is substantially constant across all timeslots.
 4. A method asclaimed in any one of the preceding claims, wherein the data callcurrently having the highest data rate has its transmit power reducedbefore another data call having a lower data rate.
 5. A method asclaimed in any one of the previous claims wherein each call has anassociated priority level, and a call having a lower priority level hasits transmit power level reduced first.
 6. A method as claimed in anyone of the previous claims wherein if the multi-carrier power amplifieris not operating at maximum transmit power, but insufficient transmitpower is available in any one timeslot, then timeslots are re-allocatedsuch that any available transmit power is re-allocated to a singletimeslot.
 7. A method as claimed in any one of the previous claims,wherein the transmit power associated with a data call has a definedminimum value.
 8. A method as claimed in claim 7, wherein the definedminimum value is equivalent to the data rate achievable using a GMSKmodulation scheme.
 9. A method as claimed in any one of the previousclaims, wherein a new call will not be set up if insufficient transmitpower is available to support the new call.
 10. A communication systemcomprising: a multi-carrier power amplifier; and a database comprising:information concerning call activity on each timeslot of each carrierassociated with the multi-carrier power amplifier; informationconcerning total transmission power of the multi-carrier power amplifierfor each timeslot; and information concerning the data rate of any datacalls supported by the multi-carrier power amplifier wherein themulti-carrier power amplifier is arranged to reduce the transmissionpower associated with a data call in order to make transmission poweravailable for a new call.